When attention is directed to a localized region within the visual field, discrimination of stimuli in that region is enhanced. Recent evidence suggests this effect may be mediated via feedback projections to visual cortex from oculomotor structures, for example the SC (Gattass & Desimone, SFN 1992) and the frontal eye fields (Moore & Fallah, PNAS 2001; Moore & Armstrong, Nature 2003). To test this idea further, we examined the effects of subthreshold microstimulation of the SC (current < eye-movement threshold) on the direction discrimination performance of two awake, behaving monkeys. Monkeys discriminated the direction of coherent motion of a localized patch of random dots positioned at one of two locations within a large, dense noise field. In humans, discrimination performance (measured as coherence threshold) was improved substantially by a spatial cue indicating the location of the patch of coherent dots. To test the SC's role in directing spatial attention, we stimulated the SC while measuring the monkeys' coherence thresholds. If SC microstimulation mediates attention, it should improve performance when the coherent patch is positioned within the movement field (MF) of the stimulated SC site, but not when the patch is remote from the SC MF. Both predictions were confirmed by the experimental results, consistent with SC playing a role in directing spatial attention.